Polymer microparticles have been conventionally used as an industrial material for various applications. Their applications include, for example, rheology control agents and matting agents for paints, modifiers for cosmetics, spacers for liquid crystal displays, shrinkage control agents for resins, column packing material, and toners. In these applications, functions developed by the polymer microparticles so used include those which are derived from raw materials that make up the microparticles and those which are achieved by the shapes themselves of the microparticles. In the case of the above-mentioned column packing materials or toners, for example, the surface properties of microparticles are important so that the function required for the microparticles relies largely upon the properties of the material that make up the microparticles. In matting agents for paints or spacers for liquid crystal displays, on the other hand, the size and shape, which the microparticles have, are themselves used as important functions as is also evident from the use of truly spherical microparticles of uniform particle size.
Conventionally-practiced production methods of polymer microparticles include a grinding method that grinds a resin to a desired particle size and a polymerization method that directly obtains polymer microparticles by emulsion polymerization or suspension polymerization. The polymerization method is useful especially as a method for obtaining spherical polymer microparticles. Many of polymer microparticles employed for industrial applications are spherical, or in particular, truly spherical in particle shape as mentioned above, and moreover, those having a narrower particle size distribution are advantageous because the effects of their addition can be obtained with a smaller amount of addition. From such a viewpoint, as a method for the production of polymer microparticles capable of meeting applications in which high functionality is required, the synthesis of microparticles by such a polymerization method as described above is considered to be a more useful method than the grinding method.
As polymer microparticles which are industrially practiced by the polymerization method these days, microparticles made of materials such as acrylic resins, polystyrene, epoxy resins, polyesters, polyamides and polyurethane can be mentioned. Of these microparticles, polyurethane microparticles are widely used primarily as a modifier for paints and coating formulations because they provide abrasion resistance, solvent resistance, heat resistance, adhesiveness and oil resistance derived from the properties of polyurethane as their base material. It is the polymerization method that industrially produces polyurethane microparticles for use as such a modifier. For example, there have been proposed a process that disperses a polyisocyanate prepolymer in water while using a dispersant and cures it by using a reaction with water (Patent Document 1) and a suspension polymerization process in a non-aqueous system where no effects are received from water (Patent Document 2).
Isocyanate compounds useful as raw materials in such production processes of polyurethane microparticles as described above are industrially produced as diverse compounds. These isocyanate compounds are, however, accompanied by a drawback in that all of them are hazardous materials and are difficult to handle. In addition, phosgene employed in the production of isocyanate compounds is a material of very strong toxicity, so that it is strongly desired to increasingly curtail its use.
On the other hand, processes that react cyclic carbonates and amines, respectively, have been reported as production processes of polyurethane resins which use no isocyanate (Patent Documents 3 and 4). These production processes are characterized in that they use no isocyanate as a raw material, and as the cyclic carbonates that are raw materials, those obtained by using carbon dioxide as a raw material are employed. Therefore, the resulting polyurethane resins are also compounds with carbon dioxide incorporated in their chemical structures. This means that the above-described technology is also a noteworthy technology in another viewpoint of contributing to the reduction of carbon dioxide as one of greenhouse gases that have become a problem in recent years.
Nonetheless, the production processes of Patent Document 3 and 4 are specialized to the production of resin solutions useful as resin binders, and no report has been made yet about a process that makes use of such a reaction for the synthesis of polyurethane microparticles.